None.
Not applicable.
1. Field of the Invention
The preferred embodiments of the present invention are directed to determining the orientation of vector sensors in well bores. More particularly, the preferred embodiments are directed to determining orientation of a series of vector sensors after installation or after movement, as well as determining calibration of the vector sensors over time.
2. Background of the Invention
It is common in the oil and gas industry to install a series of acoustic sensing devices inside the well bore, yet outside the production tubing in a hydrocarbon producing well. These acoustic sensors are used to create three-dimensional and four-dimensional surveys of the hydrocarbon producing formation of interest. The acoustic sensors are typically physically attached to a cable, and are periodically spaced along the cable. More particularly, acoustic sensors are typically housed in groups of three, with each acoustic sensor in the housing or pod responsive to acoustic signals along orthogonal axisxe2x80x94hence the term xe2x80x9cvector sensors.xe2x80x9d FIG. 1 shows an exemplary set of orthogonal axis, as well as an exemplary set of acoustic receivers 2A-C lying along those axis. If an acoustic signal is generated in the plane created by the XY axis, with no corresponding Z component, then only the acoustic sensors 2B and 2C receive a signal in response thereto. FIG. 2 exemplifies this situation. In particular, FIG. 2 shows that at an arrival time (indicated by the dashed line through all three acoustic response graphs), the signal received along the X axis (by receiver 2C) and the signal received along the Y axis (by receiver 2B) are the only axis in which acoustic signals are received. Moreover, FIG. 2 exemplifies that the source of the acoustic energy (not shown) was more closely orientated to the X axis than the Y axis as shown by the greater amplitude of the signal received along the X axis than that received along the Y axis. Thus, vector sensors have the ability to detect the orientation of an incoming signal. If the orientation of the vector sensors is known, then the orientation of the incoming signal may be calculated. Thus, knowing the orientation of each sensor pod is needed for correct operation of an acoustic or seismic system.
FIG. 3 shows a sensor cable 4 having a plurality of sensor pods 6A-E attached thereto disposed within a well bore 8. Because of the flexibility of the cable 4, the orientation of the pods 6A-E relative to each other is not known, and indeed may change during the installation process. The related art technique to determine the orientation of the sensor pods is to induce seismic or acoustic energy into the earth by a source 10 on the surface of the earth 12. In theory, the ray path of the acoustic energy created by the source 10 is confined to a plane containing both the source 10 and each respective receiver 6A-E, as indicated by the series of lines or rays 14 of FIG. 3. FIG. 4 shows an overhead view of the assumption shown in FIG. 3, indicating that the orientation of each ray 14 with respect to the borehole is assumed to be straight and known. The acoustic sensors in the sensor pods downhole receive the test signal originating from some distance from the borehole, with known orientation, and thus the orientation of the sensor pods may be calculated
However, the assumption that the ray path between the source 10 and each receiver 6A-E lies in a plane is, in most instances, incorrect. FIG. 5 exemplifies that various subsurface anomalies, such as non-horizontal formations, affect the ray paths in at least the vertical plane shown, but also in the horizontal plane. FIG. 6 shows an overhead view of the situation of FIG. 5. In particular, FIG. 6 shows that the ray path from the source 10 may shift in the horizontal plane due to subsurface anomalies. FIG. 6 also shows some subsurface formations exhibit a property known as anisotropy. In anisotropic environments, acoustic waves are broken into two orthogonal components each having slightly different propagation speeds. In the exemplary system shown in FIG. 6, the propagating acoustic wave may be broken down into two orthogonal components, indicated by dashed lines 16A and 16B. Degradation of the test signal into orthogonal components exasperates the orientation determination process.
Thus, what is needed in the art is a method and related system to determine the orientation of sensor pods that is not affected by subsurface anomalies and characteristics.
The problems noted above are solved in large part by a method and related system of determining the orientation of sensor pods placed in well bores that is not affected by subsurface anomalies and characteristics. Preferably, a signal-generating mechanism produces an acoustic signal along the casing of the well bore. The acoustic signal is detected by sensor pods and is used to determine the orientation of the sensor pods relative to the acoustic signal. If the acoustic signal does not reach all sensor pods, additional acoustic signals are generated with some overlap of sensor pods, so that their relative orientations may be determined. In this way, the absolute orientation of only one sensor pod, or the absolute orientation of only one acoustic signal in the casing, needs to be known to ascertain the orientation of all the sensor pods in the array.
Most acoustic sensing devices, as well as seismic devices generally, have as their sensing mechanism a spring loaded inertial mass. The mass in the sensing mechanism moves responsive to received energy, and the movement correspondingly creates an electric signal. In the preferred embodiments, the acoustic sensor is used in reverse, and an electric signal is applied which in turn causes the inertial mass to oscillate. This, in turn, creates an acoustic signal. Thus, each acoustic sensor in each sensor pod may be the signal generating mechanism that produces the signal detected above or below the acoustic sensor operated in this manner.
In a second aspect of the preferred embodiments, the calibration or sensitivity of acoustic sensors in each sensor pod may be tested over time. That is, the sensor pods coupling to the casing, as well as the sensitivity of the sensor pods in general, and the acoustic sensing devices within each sensor pod, may change over time. If these changes are not accounted for in the acoustic and seismic measurements, they may lead to incorrect assumptions about the state of the hydrocarbon formation monitored. In the preferred embodiments, a baseline sensor pod response is created either simultaneously with the orientation determination, or as an independent test. At later times, the same tests may be run again to determine an amount of change in responsiveness of each sensor pod or particular sensing device. In this way, the data collected in subsequent acoustic surveys may be correspondingly corrected for any change in physical characteristics of the sensor pod or the individual sensing devices, such that these errors are riot attributed to changes in formation properties.